1. Field of the Invention
The present invention relates to a projection apparatus for forming a distortion-free image of a flat plane to be scanned by means of a light deflector such as a continuously rotating mirror for example a polygonal mirror or a reciprocatingly rotating mirror for example a Galvano mirror (hereinafter collectively called rotary mirror).
2. Description of the Prior Art
There are already proposed various apparatus for projecting the image of a flat plane to be scanned onto a determined light-receiving plane by means of a light deflector such as a rotary mirror. One example of such apparatus, disclosed in the U.S. Pat. No. 3,576,368, comprises two imaging optical system and a rotary mirror provided therebetween for scanning the parallel beam. FIG. 1 shows an example of the projection apparatus utilizing such scanning system, wherein the light beam emerging from a plane 1 to be scanned is converted into a parallel beam by means of a first imaging lens 2 having a focal plane thereof on said scanned plane 1, and said parallel beam is deflected by a rotary mirror 3 and focused by a second imaging lens 4 onto a focal plane 5 thereof, so that the position P on said scanned plane 1 corresponding to a determined position P' on said plane 5 displaces with the rotation of the rotary mirror 3.
In such projection optical system, although it is possible to compensate the change in the optical path length between the scanned plane 1 and the projection plane 5 resulting from the rotation of the rotary mirror 3, there will result a change in the imaging magnification in the scanning direction (i.e. in the paper plane) and also in a direction perpendicular to said scanning direction (i.e. perpendicular to the paper plane) along with the change of the scanned position P on the scanned plane 1. Besides the rate of said change in the imaging magnification in the scanning direction is different from that in the perpendicular direction. For this reason the image formed on the focal plane 5 becomes distorted.
Also the U.S. Pat. No. 3,537,373 discloses another apparatus for scanning a flat plane 1 with a rotary mirror in which the first imaging lens 2 in FIG. 1 is eliminated and in which two lens elements in the remaining second imaging lens 4 are displaced along the optical axis thereof to compensate the change in the optical path length between the scanned plane and the focal plane 5 and to maintain the conjugate relationship between said two planes. Stated differently the apparatus disclosed in said U.S. Pat. No. 3,537,373 maintains the imaging magnification in the scanning direction of the scanned plane while maintaining the conjugate relationship between said scanned plane 1 and the focal plane 5.